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** Determination of reaction rate **

Reaction (3) is a second-order reaction (except for methyl and ethyl radicals) and has an activation energy close to zero. The rate coefficient will therefore be approximately equal to the pre-exponential factor. Various estimations and experimental determinations of its value have been made, however, a value of 10 - 1. mole". sec." appears to be an acceptable mean value for most workers [38, 58]. [Pg.269]

The reaction rate coefficient k can be determined from the experimental observation of the elimination of a model compound over time (Chapter B 4). It can be combined with literature values for kD and kR, and the experimentally determined ozone concentration to calculate the amount of OH-radicals which were present during the oxidation process. If indirect oxidation dominates, the term for the direct oxidation (kD c(03)ss) can be neglected. [Pg.130]

In other words, the experimental determinations made towards the end of a reaction contribute very little to the least squares estimate of the rate coefficient. Eqn. (68) is of little use in computation, however, unless we can make some statement about ff(ay). There are two clearcut situations which we can discuss. The ffist of these situations is that the value of ff(ay) does not depend on J and so within any one run, ff(ay) is a constant. In other words, were it possible to determine a large number of values of each ay, the width of the normal distribution would be the same in each case. Since it is only relative weights which are important in computation, we can omit such constant factors from the weights [as was done in eqn. (65)] and write [Pg.373]

The determination of absolute rate coefficients of transfer reactions of bromine atoms is much more favourable than for the corresponding reactions of fluorine or chlorine atoms. This arises because the dissociation constant of molecular bromine is high at normal experimental temperatures and the chain lengths in bromination are relatively short. The rate constant of the reaction of bromine atoms with molecular hydrogen was the first quantitative kinetic study of a radical reaction [96]. Fettis and Knox [52] evaluated the data for the Br—Hj reaction and their results are given in Table 7. Trotman-Dickenson [1] has pointed out that the subsequent data of Timmons and Weston [80] for the reaction with Hj, HD and HT are not fully compatible with the conclusions of Pettis and Knox [52]. [Pg.49]

In this, as in many catalysed reactions, the protonated substrate is postulated as an intermediate, and although the proposed reaction scheme in fact accords with all the known experimental facts it perhaps would be instructive to determine the dependence of the rate coefficient on the Hammett acidity function at high acid concentration and also to investigate the solvent isotope effect kD2JkH20. Both these criteria have been used successfully (see Sections 2.2-2.4) to confirm the intermediacy of the protonated substrate in other acid-catalysed aromatic rearrangements. [Pg.436]

From the viewpoint of the combustion chemist, mechanistic and theoretical studies of abstraction reactions serve two purposes. First, they can determine the overall rate coefficient for an abstraction over a range of temperatures, especially when there are limited experimental data. Second, the combustion modeller wishes to know the rate of abstraction at any particular site on a hydrocarbon molecule. For reaction (10) this is trivial as there is only one type of site a primary C—H bond. However, for more complex fuels there will be a variety of different sites which to a first-order approximation can be considered as primary, secondary and tertiary C—H bonds. As mentioned in the introduction to this section, Atkinson et al. [10] and Walker [11] have attempted to describe radical/ alkane kinetics with the following simple model based on equation (2.4) [Pg.146]

The diffusion coefficient for catalyst pores is usually calculated approximately. Roiter ef al. (140, 141) worked out a method for experimental determination of the diffusion coefficient and calculation of reaction rates without errors induced by diffusion. [Pg.468]

The Smoluchowski theory of diffusion-limited (or controlled) reactions relies heavily on the appropriateness of the inital condition [eqn. (3)]. Though the initial condition does not determine the steady-state rate coefficient [eqn. (20)] because diffusion of B in towards the reactant A is from large separation distances (>10nm) in the steady-state, it does directly determine the magnitude of the transient component of the rate coefficient because this is due to an excess concentration of B present initially compared with that present in the steady-state. As a first approximation to the initial distribution, the random distribution is intuitively pleasing and there is little experimental evidence available to cast doubt upon its appropriateness. Section 6.6 and Chap. 8 Sect. 2.2 present further comments on this point. [Pg.19]

The objective in developing a kinetic expression is to obtain an overall rate function that will describe the observed reaction kinetics. Determination of the rate function also involves determination of the reaction order and rate coefficients. A number of useful methods are available to derive overall rate functions from experimental data and each of these is described here. [Pg.46]

Diffusion-Controlled Reactions. The specific rates of many of the reactions of elq exceed 10 Af-1 sec.-1, and it has been shown that many of these rates are diffusion controlled (92, 113). The parameters used in these calculations, which were carried out according to Debye s theory (41), were a diffusion coefficient of 10-4 sec.-1 (78, 113) and an effective radius of 2.5-3.0 A. (77). The energies of activation observed in e aq reactions are also of the order encountered in diffusion-controlled processes (121). A very recent experimental determination of the diffusion coefficient of e aq by electrical conductivity yielded the value 4.7 0.7 X 10 -5 cm.2 sec.-1 (65). This new value would imply a larger effective cross-section for e aq and would increase the number of diffusion-controlled reactions. A quantitative examination of the rate data for diffusion-controlled processes (47) compared with that of eaq reactions reveals however that most of the latter reactions with specific rates of < 1010 Af-1 sec.-1 are not diffusion controlled. [Pg.65]

The final step of the convolution analysis is the determination of the transfer coefficient a. This coefficient, sometimes called the symmetry factor, describes how variations in the reaction free energy affect the activation free energy (equation 26). The value of a does not depend on whether the reaction is a heterogeneous or a homogeneous ET (or even a different type of reaction such as a proton transfer, where a is better known as the Bronsted coefficient). Since the ET rate constant may be described by equation (4), the experimental determination of a is carried out by derivatization of the ln/Chet-AG° and thus of the experimental Inkhei- plots (AG° = F E — E°)) (equation 27). [Pg.100]

The present paper describes the most important progress that has been made within the understanding of the atmospheric chemistry of mercury within the application of theoretical calculations and experimental studies for determination of reaction coefficients and mechanisms with halogens and other reactants. There are still large uncertainties to cope with before a reliable description of dynamics and fate of mercury can be established. Theoretical calculations represent a very cost effective method to get the first information about rate constants, reaction products and as to what experimentalists should examine. Finally, theoretical calculations can document that we actually have a full understanding of the fundamental processes of atmospheric mercury. The study of lO [53] in the Antarctic opens the possibility that 1 and lO plays an important role in the oxidation of Hg . These reaction mechanisms should continue to be studied in the field and with theoretical methods. As most laboratory studies of the oxidation mercury in the atmosphere are carried out at room temperature it is very important that theoretical calculations state the temperature dependence of the various reaction steps and the thermally stability of the reaction intermediates and end products. [Pg.54]

** Determination of reaction rate **

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